1. Technical Field
The present invention relates to a thermoelectric power module using thermoelectric elements and thereby generating electricity by utilizing difference in temperature.
2. Description of the Related Art
Conventionally, thermoelectric generation of electricity is known in which the thermoelectric power elements are disposed between a heat exchanger at a higher temperature side and another heat exchanger at a lower temperature side generate electricity. The thermoelectric power element is an application s a thermoelectric effect to be called Seebeck effect. In the case where a semiconductor material is used as a thermoelectric material, the thermoelectric power module is configured by combining at least one thermoelectric power element formed of a P-type thermoelectric material and at least one thermoelectric power element formed of an N-type thermoelectric material. Such a thermoelectric power module has a simple structure and can be easily treated, and a stable characteristic can be maintained. Therefore, research work thereof has been widely progressed toward application for the thermoelectric generation of electricity in which electricity is generated by utilizing heat in a gas discharged from an engine of a car, a furnace of a factory, and so on.
In the meantime, the thermoelectric power module is used in a temperature environment where difference between a temperature (Th) at a higher temperature portion and a temperature (Tc) at a lower temperature portion becomes as large as possible in order to obtain high thermoelectric conversion efficiency. For example, a thermoelectric power module employing a typical thermoelectric material of bismuss-tellurium (Bi—Te) system is used in a temperature environment where a temperature (Th) at the higher temperature portion becomes 280° C. at maximum.
A use for a conventional thermoelectric module is mainly cooling, and as solder for connecting electrodes to a thermoelectric element, solder of eutectic system having a composition of 37% Pb-63% Sn or lead-free solder such as Sn—Ag—Cu or the like has been used. Further, in order to prevent solder from diffusing into the thermoelectric element, nickel (Ni) or an intermetallic compound of nickel-tin (Ni—Sn) has been used (See Japanese patent application publication JP-A-10-135523). As an electrode, copper (Cu) is generally used, and in order to increase wettability for solder, a nickel film is usually formed on the electrode by means of plating or the like.
In JP-A-10-135523, it is pointed out that in a thermoelectric device using nickel as a diffusion prevention layer for preventing solder from diffusing into a thermoelectric element, a thin diffusion prevention layer having a thickness of several micro meters cannot precisely prevent the diffusion. On the other hand, it is pointed out that in the case where a thick diffusion prevention layer of nickel is used, although the thick layer of nickel strongly prevents the diffusion along a boundary layer of the block, contact strength between the thick layer of nickel and the thermoelectric element is decreased.
Accordingly, in order to make more progress of an effect of the diffusion prevention and increase strength of a structure of the diffusion prevention layer to raise reliability of the thermoelectric device, patent document 1 (JP-A-10-135523) discloses that a structure of the diffusion prevention layer of the thermoelectric device, which has N-type and P-type conductive elements and connector plates for connecting the N-type and P-type conductive elements to an electric circuit, is at least one layer made of an intermetallic compound of Ni—Sn and having a thickness of 50-3000 μm. Further, it is disclosed that a structure of the diffusion prevention layer is a metal layer consisting of a layer of an intermetallic compound of Ni—Sn and a connection layer containing tin.
On the other hand, in the case where a purpose is to generate electricity, in order to enable use of the thermoelectric module at a high temperature, it is proposed to use solder containing lead (Pb) at 85% or more as solder for connecting an electrode to a thermoelectric element (See Japanese patent application publication JP-P2009-231317A).
JP-P2009-231317A discloses a thermoelectric module in which a height of the thermoelectric module (in a height direction of a thermoelectric element) is made constant to ensure sufficient joining strength, and destruction due to overflow of solder can be prevented from occurring. This thermoelectric module consists of a P-type thermoelectric element, an N-type thermoelectric element, electrode embers, and solder for joining them, and the solder is configured of a solder substrate and particles (copper balls) Further, JP-P2009-231317A discloses that the solder substrate contains lead at 85% or more. Since the content percentage of lead in the solder is 85% or more even at a high temperature of 260° C., for example, the solder keeps joining without being melted, and therefore, the electrode members and the thermoelectric elements can be favorably joined to each other.
Further, as a diffusion prevention film of a thermoelectric element, it is proposed to use a molybdenum (Mo) film or the like (See Japanese patent application publication JP-P2008-10612A). As a solder joint layer, a nickel film has been conventionally used.
JP-P2008-10612A discloses a thermoelectric element in which a diffusion prevention layer can be formed, which layer is effective for preventing diffusion of chemical element into a thermoelectric material including at least one of bismuth, tellurium, selenium and antimony, and which layer has high peel strength. This thermoelectric element has a thermoelectric material including at least two of bismuth (Bi), tellurium (Te) selenium (Se) and antimony (Sb), a diffusion prevention layer formed on the thermoelectric material and for preventing diffusion of different elements into the thermoelectric material, and a solder joint layer for joining the diffusion prevention layer and solder to each other, wherein peel strength between the thermoelectric layer and the diffusion prevention layer or between the diffusion prevention layer and the solder joint layer is 0.6 MPa or more. Further, JP-P2008-10612A discloses that the diffusion prevention layer includes any one of molybdenum (Mo), tungsten (W) niobium (Nb) and tantalum (Ta).
However, in the high temperature environment where a temperature of a higher temperature portion exceeds 250° C., and in the case where nickel is used as a diffusion prevention layer for preventing solder from diffusing into thermoelectric element, on the contrary, there occurs a problem that nickel diffuses into a solder layer and makes segregation, and a resistance value of a joined interface increases. Alternatively, copper in an electrode diffuses via the solder layer into the thermoelectric element, and a resistance value of the thermoelectric element increases. Thereby, output power of the thermoelectric module decreases in use for a long time of 1000-2000 hours.